1. Field of the Invention
The present invention relates to an SOI semiconductor device, which in particular is capable of controlling the Kink effect, and a method of manufacturing such SOI semiconductor device.
2. Background Information
A conventional SOI-MOSFET (metal-oxide semiconductor field effect transistor) device has a structure in which transistors (MOSFETs) are formed on an SOI substrate in which a buried oxide film and a silicon film are formed on the support substrate. In this structure, each transistor has a source region, a drain region and a channel region which are formed in an active region (also called an element formation region) in the silicon film of the SOI substrate, a gate oxide film formed on the channel region, and a gate electrode formed on the gate oxide film. In the following description, the diffusion region that includes the source region, the drain region and the channel region, and the structure constituted of the gate oxide film and the gate electrode are to be referred to as a body part.
Generally, an SOI-MOSFET device is considered as having greater performance efficiency than a semiconductor device that uses a bulk substrate made of silicon. This is because each individual transistor is formed so as to be surrounded by an insulator which protects the transistor from electrical interference. In other words, since the body part of the transistor is electrically floating, a transistor is not electrically influenced by other elements. By having the body part of the transistor be in an electrically floating state, it is possible to decrease the parasitic capacitance, decrease possible leaks, or eliminate electrical interferences among transistors. As a result, it is possible to operate the transistor in a more desirable way. Performance efficiency in this case is a ratio between performance and power consumption and the like.
Moreover, in manufacturing the SOI-MOSFET device, it is possible to use a method of manufacturing a semiconductor device that uses bulk silicon (i.e. BULK-MOSFET) by applying slight changes to it. In this manufacturing method, processing steps are unchanged, but only easy layout modifications and small mask changes are required.
Accordingly, the SOI-MOSFET device can be considered as a superior semiconductor device which has achieved improved features as compared to the conventional BULK-MOSFET without having to increase its design cost.
However, with respect to the SOI-MOSFET device, since the body part of the transistor is in an electrically floating state as described above, the Kink effect, which is a phenomenon in which a drain current (Id) changes in a stepwise manner in a voltage-current characteristic (Vd-Id characteristic), may occur. Because of this, problems such as a skew being superimposed on an output signal over an input signal, especially at the time of operation by an analog signal, can occur. Furthermore, also in a digital circuit, a problem in which the circuit may operate unstably in transition can occur.
Therefore, with respect to the SOI-MOSFET device, it is a problem in that the circuit may operate unstably.
For example, Japanese Laid Open Patent Application No. 8-8431 and Japanese Laid Open Patent Application No. 11-135795 disclose some methods which cope with the problems of the Kink effect. In the methods introduced in Japanese Laid Open Patent Application No. 8-8431 and Japanese Patent Application Laid Open No. 11-135795, another diffusion region (i.e. a channel contact region) is formed in addition to a diffusion region of a transistor, and this additional diffusion region is electrically connected to a channel region by which the electric charge (i.e. electron holes or electrons) accumulated in the channel region is drawn out through the channel contact region. In this technology, the channel region and the channel contact region are electrically connected through an hourglass region formed as an extension of the channel region (q.v. Japanese Laid Open Patent Application No. 8-8431, FIG. 1) or through a low impurity density zone (q.v. Japanese Laid Open Patent Application No. 11-135795, FIG. 2) which is electrically connected with the channel region.
As mentioned above with respect to Japanese Laid Open Patent Application No. 8-8431 and Japanese Laid Open Patent Application No. 11-135795, when forming an additional channel contact region besides the diffusion region, additional structure has to be added in order to electrically connect the channel contact region with the channel region. Therefore, considering that elements have to be laid out in a limited area of a chip in particular, major changes in the entire layout is required. As a result, the all of the masks needed in a series of processes starting from the process of forming the diffusion region up to the process of forming a metal (i.e. wirings and electrodes, etc.) have to be changed, and this leads to a problem of increasing the design cost.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved SOI semiconductor device which is capable of controlling the Kink effect and which requires low cost for design, and a method of manufacturing such SOI semiconductor device. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.